The present invention generally relates to a rapid antibiotic susceptibility testing (RAST) system based on bacterial immobilization using a gelling agent, antibiotic diffusion, and tracking of single bacterial cells.
In general, the responses of cells to a drug are observed by placing the cells in a multi-well plate, injecting the drug in the form of a liquid, and monitoring time-dependent changes of the cells using an optical measurement system to obtain statistic results. As an antibiotic susceptibility testing method in a solid medium, the Kirby-Bauer (KB)-testing method, in which bacteria are scattered over an agar medium, antibiotic-absorbed papers are placed thereon and bacterial growth is observed, is known. In the case of microdilution testing in liquid media, a number of automated systems, such as VITEK2, Microscan and Phoenix, have been developed for antibiotic susceptibility testing. Such a system can be used for antibiotic susceptibility testing by placing an antibiotic in millimeter-sized wells, injecting bacteria, together with a liquid medium, into the wells, and statistically monitoring and determining the bacterial growth through turbidity.
Sepsis is one of the major causes of death in the US, necessitating rapid treatment with proper antibiotics. Roughly 750,000 patients contract severe sepsis each year in U.S. hospitals, and over 25% of them do not survive (Angus et al. (2001), Critical Care Medicine-Baltimore-, 29, 1303-1310). Conventional systems for antibiotic susceptibility testing (AST) take far too long (16-24 hr) for the timely treatment of sepsis.
When the responses of cells to different drugs are tested using the conventional systems, the cells are placed in a liquid or solid medium, the drugs are mixed with the liquid medium or drug-absorbed paper disks are placed on the solid medium to allow the cells to respond to the drugs, and the cell growth responses to the drugs are determined by turbidity (absorbance) measurement. However, such an approach is dependent on the collection of statistically valid data rather than on changes of single cells, and requires a long incubation time (usually 16-24 hours) because a predetermined number of cells should grow (usually one million cells per ml) in order to obtain statistic results. In this case, it is impossible to monitor changes occurring in single cells against drugs and monitor motile single cells in real time. Further, a great deal of time and labor is required to test the large number of drugs because the individual drugs are injected separately. The KB-test for antibiotic susceptibility testing in solid media basically requires a large number of agar medium plates to test the susceptibility of tens of antibiotics due to the limited number of the drugs that can be placed on the solid media. VITEK, an automated system developed to minimize testing time, also requires a relatively long time of about 12 hours because the turbidity of bacteria should increase above a predetermined level. Further, since environments for the conventional testing methods are different from in vivo environments, there may be many substantial differences between the test results and phenomena occurring in vivo (Gregory G. Anderson, et al. (2003), “Intracellular Bacterial Biofilm-Like Pods in Urinary Tract Infections”, Science 301, 105; Gallo et al. (2011), “Demonstration of Bacillus cereus in Orthopaedic-Implant-Related Infection with Use of a Multi-Primer Polymerase Chain Reaction-Mass Spectrometric Assay.”, J Bone Joint Surg Am, 93).